Assessing eutrophication in rivers is difficult compared to lakes and coastal waters, as nearly all algal production occurs on the riverbed. Surveying benthic algae is challenging, and variation in algal growth across space and time complicates reach-scale estimates of algal biomass. River metabolism integrates gross primary production (GPP) and ecosystem respiration (ER) to capture the processes that underlie algal growth at the reach scale. Yet, the role of algal biomass and growth form in structuring variation in GPP and ER remains poorly resolved. We explored the dynamics of reach-scale metabolism and algal biomass at six sites during the growing season in the productive Gallatin River (Montana). Sites were instrumented with dissolved oxygen sensors and selected for varying coverages of three algal growth forms—filamentous algae (Cladophora glomerata), diatomaceous mats (Didymosphenia geminata), and mixed epilithic biofilms. We surveyed transects bi-weekly to quantify percent cover and condition of each growth form and sampled all taxa to construct reach-scale biomass. Across sites, peak GPP varied in timing and magnitude but always occurred when epilithic biofilms dominated algal cover. At three of six reaches, low GPP (> 3 g O₂ m⁻² d⁻¹) for nearly two months coincided with low early-season biomass. At some sites and times, Cladophora and Didymosphenia dominated total biomass, but did not appreciably increase GPP. We observed synchrony between algal biomass and GPP, in terms of total biomass accumulation and seasonal increase in GPP, but found dominant algal growth form alone did not control GPP and ER. By integrating algal dynamics with metabolism estimates, we can derive rich information on production and establish strong baselines to monitor eutrophication in rivers.